Many hematological diseases result from aberrant chromatin and gene regulation of the maintenance, proliferation and differentiation of hematopoietic stem and progenitor cells (HSPCs). As the major H3K4 methylation enzymes in mammals, the Set1/Mll complexes occupy a crucial position in developmental biology, and are considered potential drug targets for epigenetic therapeutics due to the intimate connection of H3K4 methylation with gene expression as well as the extensive association of several subunits in these complexes with diseases, including multiple blood cancers. However, it remains unclear how the H3K4 methylation activity of the Set1/Mll complexes regulates normal and abnormal hematopoiesis. We have previously established that Dpy30, a shared subunit of the Set1/Mll complexes, facilitates chromosomal H3K4 methylation, and is crucial for efficient differentiation of embryonic stem cells by facilitating the induction of many bivalently marked developmental genes. In this proposal, we will study the functional role of Dpy30 and its associated H3K4 methylation in HSPC fate determination using a Dpy30 conditional knockout (KO) mouse model that we recently generated. We have shown that the cellular H3K4 methylation level is markedly reduced in the Dpy30 KO mice. These mice develop severe pancytopenia and have profound defects in multilineage hematopoietic reconstitution, yet strongly accumulate phenotypic early HSPCs at the expense of more downstream hematopoietic cells, suggesting a block in hematopoietic differentiation. Further analyses in a competitive transplantation system also reveal profound defects in HSPC fate determination and in expression of key regulatory genes. These results allowed us to formulate our central hypothesis in this proposal -- Dpy30 is critical for HSPC cell fate determination through facilitating H3K4 methylation of key hematopoietic genes. We thus propose two specific aims to test this hypothesis: (1): To define the role of Dpy30 in HSC maintenance and HSPC differentiation. (2): To dissect the molecular mechanisms by which Dpy30 regulates HSPC function. By revealing a previous unrecognized role of the H3K4 methylation activity of the Set1/Mll complexes in regulating HSPC fate determination, this project will have important implications for developing therapeutic strategies against certain HSPC-based hematological diseases.